More than numbers
While discussing layers, it's worth saying that the C-D-A model isn't the only model that needs to be reviewed when designing a modern DC - the Networks Team vs Firewall Team vs Wintel Team vs UNIX team lines and boundaries need to be blurred too. Especially with the likes of Flex10 and the in-enclosure Nexus kit.
Nexus isn't without it's faults though. Some of the high-availability functions of the platform are still a little rough, and the use of VSS probably extended the lifetime of Cat6500 by five years. HAving a fair bit of experience with both, I still miss the familiarity and mature feel of IOS over NX-OS, but I'd take the scalability of the Nexus hardware over Catalyst.
Also don't overlook the fact that for moderately sized data centres, you can achieve virtually the same result with 6500 as you can with Nexus but for about two thirds the cost. N7K tin especially...ouch.